docs: def _get_centered_z1pt0(self, sites):
          """
          Get z1pt0 centered on the Vs30- dependent avarage z1pt0(m)
          California and non-Japan regions
  
          """
          #: California and non-Japan regions
  
          mean_z1pt0 = (-7.15 / 4.) * np.log(((sites.vs30) ** 4. + 570.94 ** 4.)
                                             / (1360 ** 4. + 570.94 ** 4.))
          centered_z1pt0 = sites.z1pt0 - np.exp(mean_z1pt0)
  
          return centered_z1pt0